The chilling find was the latest instance of mass graves being uncovered in territory wrested from IS militants

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In this Monday, Nov. 7, 2016, frame grab from video, an Iraqi federal police officer holds a stuffed toy at the site of a mass grave in Hamam al-Alil, Iraq. Investigators are probing the mass grave that was discovered the previous day by troops advancing further into Islamic State-held territory near the city of Mosul.

Iraqi investigators were probing a mass grave on Tuesday that was discovered the previous day by troops advancing further into Islamic State-held territory near the city of Mosul, where soldiers have captured a sliver of land but later halted their advance.

The chilling find was the latest instance of mass graves being uncovered on ground wrested from IS militants. In Iraq and Syria so far, the group has killed thousands of people in extrajudicial killings, the graves a dark testimony to its brutality.

Associated Press footage from the site shows bones and decomposed bodies among scraps of clothing and plastic bags dug out of the ground by a bulldozer after Iraqi troops noticed the strong smell while advancing into the town of Hamam al-Alil on Monday.

"Investigators flew in this morning and are on their way to the grave to conduct examinations and determine the cause of death," said Cabinet official Haider Majeed, in charge of mass grave investigations.

The first officials at the site said the grave, behind an earthen embankment near an agricultural college, likely holds about 100 bodies, many of them decapitated. The town lies some 30 kilometers (19 miles) from Mosul.

It was unclear who the victims were, but a soldier at the site pulled a child's stuffed animal from the scraps of clothing and rotting flesh, swarming with flies.

"We had reports that 50 former Iraqi police officers had been killed in a building outside Mosul city," spokeswoman Ravina Shamdasani said. "This building was actually the same agricultural facility, agricultural college, that has been cited right now as the site of these mass graves."

The campaign to drive IS fighters from Mosul, Iraq's second-largest city and the extremists' last major urban stronghold in the country, began on Oct. 17.

Iraqi troops and the Kurdish peshmerga forces are now converging on Mosul, although the deepest advance into an eastern sliver of the city has stalled after militants counterattacked advancing special forces from within built-up, populated areas.

To the northeast, some 13 kilometers (8 miles) from the city, the peshmerga forces continued their push on the town of Bashiqa, believed to be largely deserted except for dozens of IS fighters.

In a report issued overnight, the U.N. human rights office said the world organization and its humanitarian partners have distributed food, water and medicine to more than 41,000 displaced people and vulnerable residents fleeing the fighting.

Since the battle for Mosul reached the city itself on Nov. 4, some 11,000 people have fled eastwards from the city, while some electricity and water supplies have been cut in eastern neighborhoods.